Several methods such as reinforcement of the edge of the hole, introduction of secondary holes, enlargement of the plate near the hole etc., have been proposed to reduce the stress concentration. Methods like the variable -thickness reinforcement of the edge have also been adopted to obtain more uniform distribution around the periphery of the hole. All these methods involve either addition of material or introduction of another discontinuity. But it was proposed and validated [1] through photo-elastic model that optimising the shape of the discontinuity will not only decrease the stress concentration but also the weight and consequent increment in the fatigue life of the structure.

Irrespective of type of holes, cracks, usually, start as corner cracks or as surface flaws from wall [2]. Moreover, such cracks fall in the category of short cracks [3]. Due to high stress concentration, the initial growth rate of a short crack will be greater than (or at least equal to) the corresponding growth rate of a long crack. In most of the cases, very little life would be left as soon as the crack from the notch becomes a through crack. Consequently studies of such short cracks are of considerable importance.

In order to detect such cracks in a component in service, the most desirable procedure from view point of structural integrity examination and inspection is to detect initiation by on-line monitoring. Acoustic emission, a highly sensitive technique appears to be particularly well suited for this type of nondestructive examination. However, for using this technique effectively, it is necessary to establish correlation between acoustic emission parameters and fatigue crack initiation and propagation in a variety of situations.

In the present investigation, a specific example was considered. Acoustic emission (AE) was monitored during fatigue tests on A1-Cu alloy sheets with circular and optimised holes to correlate AE activity with the initiation and propagation of cracks from two types of discontinuities. The correlation of an AE parameter with stress concentration was studied in both the cases of optimised as well as circular hole. The results obtained during AE monitoring indicate the existence of three stages relating to the initiation and propagation of a crack from the hole boundary. The significance of the three stages indicated in AE data are discussed in this paper. REFERENCES

1. A. J. Durelli and K. Rajaiah, "Lighter and Stronger", Experimental Mechanics, Vol. 20, pp. 369-380, 1980
2. D. M. Broek, "Cracks at Structural Holes", Metals and Ceramics Information Centre, Battelle Columbus Laboratories, Ohio, 1975
3. J. C. Newman, Jr., M. H. Swain and E. P. Phillips, An Assessment of the Small - Crack Effect for s2024-T3 Aluminium Alloy", Proc. Conf. On Small Fatigue Cracks, California, pp. 427-452, 1986